Piezoelectric actuator

ABSTRACT

A piezoelectric actuator has a piezoelectric element and a frictional portion. The piezoelectric element simultaneously generates first and second vibration modes in response to a voltage applied thereto. The frictional portion is arranged on an outer surface of the piezoelectric element and adapted to come into contact with a body to be driven and cause a frictional force therewith. The frictional portion has a glass-containing portion containing a glass material and being aimed such as to project from the outer surface of the piezoelectric element. Without restricting the size of the piezoelectric element, the piezoelectric actuator inhibits its driving state from fluctuating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric actuator.

2. Related Background Art

Known as a piezoelectric actuator is one comprising a piezoelectric bodyadapted to simultaneously generate first and two vibration modes inresponse to an electric power applied thereto and a frictional memberwhich is separate from the piezoelectric body and partly secured to aconcave portion formed as a depression on one side of the piezoelectricbody (see, for example, Japanese Patent Application Laid-Open No.2008-99549).

SUMMARY OF THE INVENTION

The piezoelectric actuator described in Japanese Patent ApplicationLaid-Open No. 2008-99549 has problems as follows.

The piezoelectric element (piezoelectric body) has active and inactiveportions which are adapted to be displaced and not, respectively, when avoltage is applied thereto. Since it is necessary for the concaveportion to be formed in the inactive portion so as not to affect theactive portion, the inactive portion is restricted in its designed sizessuch as the width (thickness) thereof. That is, the width (thickness) ofthe inactive portion must be set greater than the depth of the concaveportion, which inhibits the piezoelectric element from becoming smallerin size.

The frictional member is typically secured to the concave portion bymaking the inner form of the concave portion greater than the outer formof the frictional member and then fixing the frictional member to theinner surface of the concave portion with an adhesive or the like. Sincethe inner form of the concave portion is greater than the outer form ofthe frictional member, the position at which the frictional member isfixed to the concave portion may vary.

The piezoelectric element (piezoelectric body) has two resonance modes,i.e., first and second vibration modes. When the positions of frictionalmembers vary among piezoelectric actuators, the resonance frequencies intheir resonance modes fluctuate. This affects the magnitude ofvibrations (amplitude) of piezoelectric elements, whereby the drivingstate varies among the piezoelectric actuators. The inventors conductedresearches and studies and, as a result, have found that, when a bendingvibration mode in a thickness direction of the piezoelectric element(piezoelectric body) is included as a resonance mode, the position ofthe frictional member extremely affects the resonance frequency in thebending vibration mode.

It is an object of the present invention to provide a piezoelectricactuator which, without restricting the size of its piezoelectricelement, can inhibit its driving state from fluctuating.

The piezoelectric actuator in accordance with the present inventioncomprises a piezoelectric element adapted to simultaneously generatefirst and second vibration modes in response to a voltage appliedthereto, and a frictional portion arranged on an outer surface of thepiezoelectric element and adapted to come into contact with a body to bedriven and cause a frictional force therewith; wherein the frictionalportion has a glass-containing portion containing a glass material andbeing formed such as to project from the outer surface of thepiezoelectric element.

Since the glass-containing portion is formed on the outer surface of thepiezoelectric element such as to project from the outer surface, theforming of the glass-containing portion will not affect the designedsize of the inactive portion in the piezoelectric actuator in accordancewith the present invention. This can prevent the frictional portion(glass-containing portion) from restricting the size of thepiezoelectric element.

In the present invention, it is not necessary for the piezoelectricelement to be formed with a concave portion as in the piezoelectricactuator described in Japanese Patent Application Laid-Open No.2008-99549. This can eliminate the variation in the position of thefrictional portion caused by the forming of the concave portion, therebyinhibiting the resonance frequency from fluctuating in resonance modes(first and second vibration modes). As a result, the driving state ofthe piezoelectric actuator can be kept from varying.

The frictional portion may further have a protective film which is madeof a material harder than the glass-containing portion and covers anouter surface of the glass-containing portion. The protective film maycontain DLC, TiN, SiC, or BP. This can protect the glass-containingportion against damages and the like and prevent the glass-containingportion from peeling, thereby appropriately transmitting thedisplacement of the piezoelectric element to the body to be driven.

The glass material contained in the glass-containing portion may bedispersed in the piezoelectric element. This can improve the connectionstrength between the piezoelectric element and the glass-containingportion, thereby preventing the frictional portion from peeling and soforth.

The present invention can provide a piezoelectric actuator which,without restricting the size of its piezoelectric element, can inhibitits driving state from fluctuating.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not to beconsidered as limiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the multilayer piezoelectric actuator inaccordance with an embodiment of the present invention;

FIG. 2 is a sectional view taken along the line II-II of FIG. 1;

FIG. 3 is an exploded perspective view of the multilayer piezoelectricactuator 1 illustrated in FIG. 1;

FIG. 4 is a view showing vibration modes of the multilayer piezoelectricactuator in accordance with the embodiment;

FIG. 5 is a view showing how the multilayer piezoelectric actuator inaccordance with the embodiment drives a rotor;

FIG. 6 is a flowchart showing a method of manufacturing the multilayerpiezoelectric actuator in accordance with the embodiment;

FIG. 7 is a perspective view showing a multilayer body obtained by alamination and press process;

FIG. 8 is a perspective view showing a multilayer body and glasspatterns (glass-containing portions) obtained by a glass-containingportion formation process;

FIG. 9 is a view showing respective magnitudes of displacement in thepiezoelectric actuator in accordance with the embodiment and aconventional multilayer piezoelectric actuator at the time of driving;and

FIG. 10 is a sectional view showing the multilayer piezoelectricactuator in accordance with a modified example of the embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, preferred embodiments of the present invention will beexplained in detail with reference to the accompanying drawings. In theexplanation, the same constituents or those having the same functionswill be referred to with the same signs, while omitting theiroverlapping descriptions.

With reference to FIGS. 1 to 3, the structure of the multilayerpiezoelectric actuator in accordance with an embodiment of the presentinvention will be explained. FIG. 1 is a perspective view of themultilayer piezoelectric actuator in accordance with this embodiment.FIG. 2 is a sectional view taken along the line II-II of FIG. 1. FIG. 3is an exploded perspective view of the multilayer piezoelectric actuator1 shown in FIG. 1. FIGS. 2 and 3 depict a dash-single-dot line CL at thelongitudinal center of the multilayer piezoelectric actuator. In FIG. 3,through-hole conductors are assumed to be formed at positions wherevertically extending dotted lines pass, while through-holes are so smallthat they are not depicted.

As shown in FIG. 1, the multilayer piezoelectric actuator 1 functions tobe displaced in response to an AC voltage applied thereto, so as to movea body to be driven 3 (e.g., rotor). The multilayer piezoelectricactuator 1 comprises a substantially rectangular parallelepipedpiezoelectric element 2 formed by laminating and integrating a pluralityof piezoelectric layers together and frictional portions 4A, 4B adaptedto come into contact with the body to be driven 3 and cause a frictionalforce therewith. The frictional force lets the body to be driven 3 move.

The piezoelectric element 2 of the multilayer piezoelectric actuator 1has an oblong first principal surface (upper face in FIG. 1) 2 a and anoblong second principal surface (lower face in FIG. 1) 2 b. In thefollowing explanation, the longer- and shorter-side directions of thefirst and second principal surfaces 2 a, 2 b and the direction alongwhich the first and second principal surfaces 2 a, 2 b oppose eachother, i.e., the laminating direction of piezoelectric layers, will bereferred to as longitudinal, width, and thickness directions of thepiezoelectric element 2, respectively. The piezoelectric element 2 isset to have sizes on the order of 1 to 20 mm, 1 to 10 mm, and 0.2 to 5mm in the longitudinal, width, and thickness directions, respectively.

An outer electrode layer 11 is disposed at the longitudinal centerposition of the first principal surface 2 a of the piezoelectric element2. The outer electrode layer 11 serves as terminal electrodes whenmounting another external device thereon. The outer electrode layer 11is arranged in the width direction at the longitudinal center positionof the first principal surface 2 a. The outer electrode layer 11includes a first outer electrode 12, a ground outer electrode 13, and asecond outer electrode 14 which are electrically insulated from eachother on the first principal surface 2 a.

The first outer electrode 12 is located on one end side of the widthdirection and has a rectangular form. The first outer electrode 12 isconnected to a voltage output terminal of the external device. Theground outer electrode 13 is located at substantially the center in thewidth direction and has a rectangular form. The ground outer electrode13 is connected to a ground terminal of the external device. The secondouter electrode 14 is located on the other end side of the widthdirection and has a rectangular form. The second outer electrode 14 isconnected to a voltage output terminal of the external device. The firstouter electrode 12, ground outer electrode 13, and second outerelectrode 14 are formed by applying an electrically conductive paste tothe first principal surface 2 a of the piezoelectric element 2 andsintering the paste at a predetermined temperature (e.g., about 700°C.). The sintered electrode layer obtained by sintering the electricallyconductive paste may further be subjected to electroplating, so as toform a plating layer thereon. As the electrically conductive paste, onecontaining an electrically conductive material mainly composed of Ag maybe used. An example of the plating layer is an Ni/Au plating layer.

The outer electrodes 12, 13, 14 may also be formed by sputtering, vapordeposition, or the like using a metal mask formed with respectiveopenings at portions corresponding to the outer electrodes 12, 13, 14.Examples of the film structure constituting the outer electrodes 12, 13,14 in this case include Cr/Ni, NiCu/Ag, SnAg, and Au.

The frictional portions 4A, 4B are disposed on the second principalsurface 2 b of the piezoelectric element 2 and cause a frictional forcewith the body to be driven 3. The frictional portions 4A, 4B haveglass-containing portions 4A₁, 4B₁ and a protective film 5. Theglass-containing portions 4A₁, 4B₁ contain a glass material. As theglass material to be contained, soda glass, crystal glass, borosilicateglass, and the like can be used. The glass-containing portions 4A₁, 4B₁may further contain metal materials such as Ag and AgPd, ceramicmaterials such as ZrO₂ and Al₂O₃, metal oxide materials such as MgO andCaO, and the like in addition to the glass material mentioned above.

The glass-containing portions 4A₁, 4B₁ are arranged on the secondprincipal surface 2 b of the piezoelectric element 2 such as to projectfrom the second principal surface 2 b. Each of the glass-containingportions 4A₁, 4B₁ has a substantially semicircular cylindrical form suchas to yield a substantially semicircular cross section when cut along aplane orthogonal to the longitudinal direction of the piezoelectricelement 2. Letting L be the longitudinal size of the piezoelectricelement 2, the glass-containing portion 4A₁ is disposed such as toextend widthwise of the piezoelectric element 2 at a position separatedby L/3 from one longitudinal end 2 c of the piezoelectric element 2. Theglass-containing portion 4B₁ is disposed such as to extend widthwise ofthe piezoelectric element 2 at a position separated by L/3 from theother longitudinal end 2 d of the piezoelectric element 2 (see FIG. 2).In this embodiment, the length of each of the glass-containing portions4A₁, 4B₁ is the same as the width of the piezoelectric element 2.

The protective film 5 is made of a material harder than theglass-containing portions 4A₁, 4B₁, examples of which include DLC(Diamond-Like Carbon), TiN, SiC, and BP (Boron Phosphide). Theprotective film 5 covers the second principal surface 2 b and theglass-containing portions 4A₁, 4B₁. The protective film 5 is not alwaysrequired to cover the second principal surface 2 b but may be formedsuch as to cover at least the glass-containing portions 4A₁, 4B₁. Thethickness of the protective film 5 is on the order of 0.1 to 20 μm, forexample.

As shown in FIGS. 2 and 3, the piezoelectric element 2 is constructed asa multilayer body in which rectangular sheet-like piezoelectric layers10, 20, 30, 40, 50, 60, 70 having a piezoelectric characteristic (i.e.,adapted to deform when energized), a relay electrode 21, a first innerelectrode 31, a first ground electrode layer (ground electrode layer)41, and a second inner electrode layer 51 are laminated. The relayelectrode layer 21, first inner electrode layer 31, first groundelectrode layer (ground electrode layer) 41, and second inner electrodelayer 51 are arranged one by one along the laminating direction of thepiezoelectric layers 10, 20, 30, 40, 50, 70 (i.e., thickness directionof the piezoelectric element 2) within the piezoelectric element 2.

The outer electrode layer 11 is formed on the piezoelectric layer 10serving as the first principal surface 2 a of the piezoelectric element2 as will be explained later. The relay electrode layer 21, first innerelectrode layer 31, first ground electrode layer 41, and second innerelectrode layer 51 are formed on the piezoelectric layers 20, 30, 40,and 50, respectively.

Each of the piezoelectric layers 10, 20, 30, 40, 50, 70 is made of apiezoelectric ceramic material mainly composed of PZT, for example. Eachof the piezoelectric layers 10, 20, 30, 40, 50, 70 has a thickness onthe order of 10 to 100 μm, for example. In the multilayer piezoelectricactuator 1 in practice, the plurality of piezoelectric layers 10, 20,30, 40, 50, 70 are integrated to such an extent that their boundariesare indiscernible.

The relay electrode layer 21 is arranged widthwise of the piezoelectriclayer 20 at the longitudinal center position thereof. The relayelectrode layer 21 includes a first relay electrode 22, a ground relayelectrode 23, and a second relay electrode 24 which are electricallyinsulated from each other on the piezoelectric layer 20. The first relayelectrode 22 is located on one end side of the width direction and has arectangular form. The ground relay electrode 23 is located at the centerof the width direction and has a rectangular form whose longitudinaldirection is oriented in the width direction. The second relay electrode24 is located on the other end side of the width direction and has arectangular form.

The first inner electrode layer 31 includes a first electrode 32, aground relay electrode 33, and a second electrode 34 which areelectrically insulated from each other on the piezoelectric layer 30.The ground relay electrode 33 is positioned such as to overlap theground relay electrode 23 at the longitudinal center position as seenfrom the thickness direction and has a rectangular form.

The first electrode 32 has an electrode portion 32 a and a relayelectrode portion 32 b. The electrode portion 32 a is disposed closer toone end 2 c (i.e., closer to one end 30 c of the piezoelectric layer 30)than is the longitudinal center position of the piezoelectric element 2.The relay electrode portion 32 b is disposed at the longitudinal center.The electrode portion 32 a is positioned such as to cover the upper faceof the piezoelectric layer 30 in substantially all of the area extendingfrom the ground relay electrode 33 to one end 30 c and has a rectangularform. The relay electrode portion 32 b is positioned such as to projectfrom the electrode portion 32 a to the longitudinal center position andoverlap the first relay electrode 22 as seen from the thicknessdirection and has a rectangular form.

The second electrode 34 has an electrode portion 34 a and a relayelectrode portion 34 b. The electrode portion 34 a is disposed closer tothe other end 2 d (i.e., closer to the other end 30 d of thepiezoelectric layer 30) than is the longitudinal center position of thepiezoelectric element 2. The relay electrode portion 34 b is disposed atthe longitudinal center. The electrode portion 34 a is positioned suchas to cover the upper face of the piezoelectric layer 30 insubstantially all of the area extending from the ground relay electrode33 to the other end 30 d and has a rectangular form. The relay electrodeportion 34 b is positioned such as to project from the electrode portion34 a to the longitudinal center position and overlap the second relayelectrode 24 as seen from the thickness direction and has a rectangularform.

The first ground electrode layer 41 includes a first relay electrode 42,a ground electrode 43, and a second relay electrode 44 which areelectrically insulated from each other on the piezoelectric layer 40.The ground electrode 43 is positioned such as to cover substantially thewhole surface of the upper face of the piezoelectric layer 40 and has asubstantially rectangular form. The ground electrode 43 is disposed suchas to overlap all of the electrode portions 32 a, 34 a and ground relayelectrodes 23, 33 as seen from the thickness direction. At thelongitudinal center position of the ground electrode 43, rectangularrecesses are formed on both end sides of the width direction of thepiezoelectric layer 40. The first and second relay electrodes 42, 44 aredisposed at the recesses, respectively. The first relay electrode 42 ispositioned such as to overlap the relay electrode portion 32 b and thefirst relay electrode 22 as seen from the thickness direction. Thesecond relay electrode 44 is positioned such as to overlap the relayelectrode portion 34 b and the second relay electrode 24 as seen fromthe thickness direction.

The second inner electrode layer 51 includes a third electrode 54, aground relay electrode 53, and a fourth electrode 52 which areelectrically insulated from each other on the piezoelectric layer 50.The ground relay electrode 53 is positioned such as to overlap theground relay electrodes 23, 33 at the longitudinal center position asseen from the thickness direction and has a rectangular form.

The third electrode 54 has an electrode portion 54 a and a relayelectrode portion 54 b. The electrode portion 54 a is disposed closer toone end 2 c (i.e., closer to one end 50 c of the piezoelectric layer 50)than is the longitudinal center position of the piezoelectric element 2.The relay electrode portion 54 b is disposed at the longitudinal center.The electrode portion 54 a is positioned such as to cover the upper faceof the piezoelectric layer 50 in substantially all of the area extendingfrom the ground relay electrode 53 to one end 50 c and has a rectangularform. As a consequence, the electrode portion 54 a overlaps theelectrode portion 32 a and a portion of the ground electrode 43 as seenfrom the thickness direction. The relay electrode portion 54 b has arectangular form such as to project from the electrode portion 54 a tothe longitudinal center position and overlap the second relay electrodes24, 44 and the relay electrode portion 34 b as seen from the thicknessdirection.

The fourth electrode 52 includes an electrode portion 52 a and a relayelectrode portion 52 b. The electrode portion 52 a is disposed closer tothe other end 2 d (i.e., closer to the other end 50 d of thepiezoelectric layer 50) than is the longitudinal center position of thepiezoelectric element 2. The relay electrode portion 52 b is disposed atthe longitudinal center. The electrode portion 52 a is positioned suchas to cover the upper face of the piezoelectric layer 50 insubstantially all of the area extending from the ground relay electrode53 to the other end 50 d and has a rectangular form. As a consequence,the electrode portion 52 a overlaps the electrode portion 34 a and aportion of the ground electrode 43 as seen from the thickness direction.The relay electrode portion 52 b is positioned such as to project fromthe electrode portion 52 a to the longitudinal center position andoverlap the first relay electrodes 22, 42 and relay electrode portion 32b thereabove as seen from the thickness direction and has a rectangularform.

Through-holes penetrating through the piezoelectric layers 10, 20, 30,40 in their thickness direction are formed at their positionscorresponding to the first relay electrodes 22, 42 and relay electrodeportions 32 b, 52 b. A first through-hole conductor 6 is disposed inthese through-holes. The first through-hole conductor 6 electricallyconnects the first outer electrode 12, first relay electrode 22, firstelectrode 32, first relay electrode 42, and fourth electrode 52together.

Through-holes penetrating through the piezoelectric layers 10, 20, 30,40 in their thickness direction are formed at their positionscorresponding to the ground relay electrodes 23, 33, 53 and thelongitudinal center region of the ground electrode 43. A groundthrough-hole conductor 7 is disposed in these through-holes. The groundthrough-hole conductor 7 electrically connects the ground outerelectrode 13, ground relay electrodes 23, 33, ground electrode 43, andground relay electrode 53 together.

Through-holes penetrating through the piezoelectric layers 10, 20, 30,40 in their thickness direction are formed at their positionscorresponding to the second relay electrodes 24, 44 and relay electrodeportions 34 b, 54 b. A second through-hole conductor 8 is disposed inthese through-holes. The second through-hole conductor 8 electricallyconnects the second outer electrode 14, second relay electrode 24,second electrode 34, second relay electrode 44, and third electrode 54together.

The first through-hole conductor 6, ground through-hole conductor 7, andsecond through-hole conductor 8 are located at the longitudinal centerposition of the piezoelectric element 2 and are arranged in a row alongthe width direction of the piezoelectric element 2. The through-holeconductors 6, 7, 8 contain an electrically conductive material. Theelectrically conductive material contained in the through-holeconductors 6, 7, 8 may be at least one kind of metals selected from thegroup consisting of Pd, Ag, Cu, W, Mo, Sn, and Ni or an alloy containingat least one kind of these metals. Each of the through-hole conductors6, 7, 8 has a diameter on the order of 20 to 100 μm, for example. Thepiezoelectric element 2 constituted by the electrode layers 11, 21, 31,41, 51 and piezoelectric layers 10, 20, 30, 40, 50, 70 includesthickness adjustment layers LY1, LY2 and a driving layer LY3 forvibrating the piezoelectric element 2. The driving layer LY3 includes anactive portion, while each of the thickness adjustment layers LY1, LY2includes an inactive portion.

The driving layer LY3 is constituted by the first inner electrode layer31, piezoelectric layer 30, first ground electrode layer 41,piezoelectric layer 40, and second inner electrode layer 51. Thethickness adjustment layer LY1 is constituted by the piezoelectric layer10, relay electrode layer 21, and piezoelectric layer 20. The thicknessadjustment layer LY2 is constituted by the piezoelectric layers 50, 70.At the time of manufacturing the piezoelectric element 2, the thicknessadjustment layers LY1, LY2 are polished so as to adjust their thickness,thereby regulating the vibration frequency in the piezoelectric element2. The thickness adjustment layers LY1, LY2 are set to substantially thesame thickness, whereby the piezoelectric element 2 has a structuresymmetrical about the thickness direction.

In the driving layer LY3, the piezoelectric layer 30 held between thefirst inner electrode layer 31 and the first ground electrode layer 41and the piezoelectric layer 40 held between the first ground electrodelayer 41 and the second inner electrode layer 51 have been depolarized.The depolarization is effected from the first inner electrode layer 31to the first ground electrode layer 41 and from the second innerelectrode layer 51 to the first ground electrode layer 41.

Operations of the multilayer piezoelectric actuator 1 will now beexplained with reference to FIGS. 4 and 5. FIG. 4 is a view showingvibration modes of the multilayer piezoelectric actuator 1. FIG. 5 is aview showing how the multilayer piezoelectric actuator 1 drives the bodyto be driven 3.

The multilayer piezoelectric actuator 1 has two resonance modes at thetime of driving. Specifically, the multilayer piezoelectric actuator 1vibrates according to a longitudinal vibration mode of vibratinglongitudinally of the piezoelectric element 2 as shown in FIG. 4( a) anda bending vibration mode in the thickness direction of the piezoelectricelement 2 as shown in FIG. 4( b) which are superposed on each other. Therespective resonance frequencies in the longitudinal and bendingvibration modes have been fitted to each other by polishing thethickness adjustment layers LY1, LY2 of the piezoelectric element 2 (seeFIG. 2).

FIG. 5 shows a state in which the longitudinal and bending vibrationmodes are superposed on each other. When an active portion A1constructed by the first electrode 32, ground electrode 43, anddielectric layer 30 and an active portion A4 constructed by the fourthelectrode 52, ground electrode 43, and piezoelectric layer 40 aredriven, as illustrated in FIG. 5( a), the frictional portion 4B comesinto contact with the body to be driven 3, thereby causing a frictionalforce between the frictional portion 4B and the body to be driven 3. Thefrictional force generated between the frictional portion 4B and thebody to be driven 3 lets the body to be driven 3 move in the arroweddirection in FIG. 5( a).

When an active portion A2 constructed by the second electrode 34, groundelectrode 43, and piezoelectric layer 30 and an active portion A3constructed by the third electrode 54, ground electrode 43, andpiezoelectric layer 40 are driven, as shown in FIG. 5( b), thefrictional portion 4A comes into contact with the body to be driven 3,thereby causing a frictional force between the frictional portion 4A andthe body to be driven 3. The frictional force generated between thefrictional portion 4A and the body to be driven 3 lets the body to bedriven 3 move in the arrowed direction in FIG. 5( b).

When the piezoelectric element 2 is driven by applying voltages whosephases shift from each other by 90° to the first and second outerelectrodes 12, 14, respectively, elliptic motions whose phases shiftfrom each other by 180° occur in the frictional portions 4A, 4B,respectively. As a consequence, the frictional forces alternately act onthe body to be driven 3, whereby the body to be driven 3 moves.

Referring to FIGS. 2 and 5, the above-mentioned vibrations have nodes(where no amplitudes occur in the vibrations) at the longitudinal centerposition of the piezoelectric element 2, a position separated from oneend 2 c by L/6, and a position separated from the other end 2 d by L/6,respectively. One end 2 c, the other end 2 d, a position separated fromone end 2 c by L/3 (i.e., the position where the frictional portion 4Ais disposed), and a position separated from the other end 2 d by L/3(i.e., the position where the frictional portion 4B is disposed) arepositions where the maximum amplitude occurs.

A method of manufacturing the multilayer piezoelectric actuator 1 inaccordance with this embodiment will now be explained with reference toFIG. 6. FIG. 6 is a flowchart showing the method of manufacturing themultilayer piezoelectric actuator 1 in accordance with this embodiment.

As shown in FIG. 6, the process for manufacturing the multilayerpiezoelectric actuator 1 begins with a coating preparation process S1.The coating preparation process S1 mixes a piezoelectric material formaking the piezoelectric layers 70, an organic solvent, and an organicbinder together, so as to turn them into a coating material.Subsequently, a sheet preparation process S2 is carried out. The sheetpreparation process S2 applies the coating material obtained by thecoating preparation process S1 to PET films, so as to form piezoelectricsheets each having a thickness corresponding to that of eachpiezoelectric layer.

After the sheet preparation process S2, a through-hole formation processS3 is carried out. The through-hole formation process S3 forms thepiezoelectric sheets with through-holes at predetermined positionscorresponding to positions where the through-hole conductors 6, 7, 8 arearranged. After forming the piezoelectric sheets with the through-holes,an inner electrode printing process S4 is carried out.

Using an electrically conductive paste, the inner electrode printingprocess S4 forms the upper faces of uncut piezoelectric layers 10 to 70with their corresponding electrode patterns and through-hole conductorpatterns by screen printing or the like. Here, the piezoelectric layers10 to 70 are formed with patterns corresponding to the outer electrodelayer 11, relay electrode layer 21, first inner electrode layer 31,first ground electrode layer 41, second inner electrode layer 51, andthrough-hole conductors 6, 7, 8. After forming the electrode patterns,the lamination and press process S5 is carried out.

The lamination and press process S5 laminates the uncut piezoelectriclayers 10, 20, 30, 40, 50, 70 in this order from the upper side andpresses them. This yields a multilayer body 80 as shown in FIG. 7. Afterthe pressing, a debindering and firing process S6 is carried out. Here,under a predetermined heat treatment condition, the multilayer body 80is debindered and fired.

After firing the multilayer body 80, a polishing process S7 is carriedout. The polishing process S7 polishes the thickness adjustment layersLY1, LY2, so as to fit the respective resonance frequencies of thelongitudinal and bending vibration modes to each other. Specifically,the piezoelectric layer 10 or 70 is polished. After polishing, aglass-containing portion formation process S8 is carried out.

The glass-containing portion formation process S8 prepares a glass pastecontaining the above-mentioned glass material. Then, the glass paste isapplied to desirable positions on the outer surface constituted by thepiezoelectric layer 70 in the multilayer body 80 as shown in FIG. 8.This forms glass patterns 82 on the outer surface of the multilayer body80. The glass patterns 82 are formed on the outer surface of themultilayer body 80 such as to extend in directions corresponding to thewidth directions of the piezoelectric elements 2. Each glass pattern 82has a height on the order of 10 to 200 μm, for example. Each glasspattern 82 has a width on the order of 50 to 700 μm, for example.

The glass paste is obtained by mixing a powder of the above-mentionedglass material, an organic binder, and an organic solvent. The glasspaste may also contain any of powders of the above-mentioned metal,ceramic, and metal oxide materials. The glass paste may be applied by adispenser, screen printing, or the like. Thereafter, under apredetermined heat treatment condition, the glass paste is firedtogether with the multilayer body 80. This forms the glass-containingportions 4A₁, 4B₁ on the multilayer body 80. At this time, the glassmaterial contained in the glass paste partly diffuses into themultilayer body 80 (piezoelectric elements 2), thereby improving theconnection strength between the multilayer body 80 (piezoelectricelements 2) and the glass-containing portions 4A₁, 4B₁. Each of theglass-containing portions 4A₁, 4B₁ has a height on the order of 10 to200 μm, for example, and a width on the order of 50 to 700 μm, forexample.

The glass paste is applied by the dispenser or screen printing with apositional precision much higher than a machining precision with whichthe multilayer body 80 is formed with a concave portion. This improvesthe positional precision for forming the glass-containing portions 4A₁,4B₁ and thus can prevent the glass-containing portions 4A₁, 4B₁ fromshifting their positions. When the glass paste is applied by thedispenser or screen printing, the height of the glass patterns 82 can bekept low as mentioned above, whereby the multilayer piezoelectricactuator 1 can attain a lower profile. The application by the dispenseror screen printing is a known process, which can easily form theglass-containing portions 4A₁, 4B₁ at a very low cost.

Subsequently, a surface electrode formation process S9 is carried out.Here, using the electrically conductive paste, electrode patternscorresponding to the first outer electrode 12, ground outer electrode13, and second outer electrode 14 for electrically connecting thethrough-hole conductors 6, 7, 8 exposed at the upper face of thepiezoelectric layer 10 to external circuits are formed on the upper faceof the piezoelectric layer 10 by screen printing or the like.Thereafter, the electrode patterns are sintered under a predeterminedheat treatment condition. This forms the first outer electrode 12,ground outer electrode 13, and second outer electrode 14. The outerelectrodes 12, 13, 14 may be formed by sputtering, vapor deposition, orthe like through a metal mask as mentioned above.

Next, a protective film formation process S10 is carried out. Theprotective film formation process S10 forms the protective film 5 so asto cover the outer surface of the multilayer body 80 formed with theglass-containing portions 4A₁, 4B₁ and the glass-containing portions4A₁, 4B₁. For forming the protective film 5, plasma CVD or the like canbe used.

Next, a depolarization process S11 is carried out. The depolarizationprocess S11 performs depolarization by a known technique in order forpiezoelectric materials to attain fixed polarities. Thereafter, a piececutting process S12 for cutting the multilayer body 80 intopiezoelectric elements 2 is carried out. The multilayer body 80 is cutalong dotted lines shown in FIGS. 7 and 8. The individually cutpiezoelectric elements 2 are put into a barrel polishing machine, so asto perform a barrel polishing process S13. Finally, an electriccharacteristic inspection process S14 for inspecting electriccharacteristics and an exterior inspection process S15 are carried out,whereby the manufacturing process shown in FIG. 6 ends.

Operations and effects of thus constructed multilayer piezoelectricactuator 1 will now be explained.

As mentioned above, the multilayer piezoelectric actuator 1 comprises arectangular parallelepiped element body, formed by laminating aplurality of piezoelectric layers, having first and second principalsurfaces; a first inner electrode layer, arranged within the elementbody so as to oppose the first and second principal surfaces, having afirst electrode including an electrode portion arranged closer to oneend than is a center position in a longitudinal direction of the elementbody and a second electrode, electrically insulated from the firstelectrode, including an electrode portion arranged closer to the otherend than is the center position; a ground electrode layer having aground electrode arranged within the element body so as to oppose theelectrode portions of the first and second electrodes of the first innerelectrode layer through the piezoelectric layer on the second principalsurface side; a second inner electrode layer, arranged within theelement body so as to oppose the ground electrode layer through thepiezoelectric layer, having a third electrode including an electrodeportion arranged closer to one end than is the center position and afourth electrode, electrically insulated from the third electrode,including an electrode portion arranged closer to the other end than isthe center position; an outer electrode layer, formed on the firstprincipal surface, having a first outer electrode, a second outerelectrode, and a ground outer electrode which are electrically insulatedfrom each other; a first through-hole conductor extending in a thicknessdirection which is a laminating direction of the piezoelectric layerswithin the element body so as to electrically connect the first outerelectrode, first electrode, and fourth electrode to each other; a secondthrough-hole conductor extending in the thickness direction within theelement body so as to electrically connect the second outer electrode,second electrode, and third electrode to each other; and a groundthrough-hole conductor extending in the thickness direction within theelement body so as to electrically connect the ground outer electrodeand ground electrode to each other; the first through-hole conductor,second through-hole conductor, and ground through-hole conductor areplaced at the center position and arranged in a row along the widthdirection of the element body; and the piezoelectric actuator 1generates a longitudinal vibration in its longitudinal direction and abending vibration in its thickness direction.

In the multilayer piezoelectric actuator 1 in accordance with thisembodiment, the first through-hole conductor 6 for electricallyconnecting the first outer electrode 12, first electrode 32, and fourthelectrode 52 to each other within the piezoelectric element 2, thesecond through-hole conductor 8 for electrically connecting the secondouter electrode 14, second electrode 34, and third electrode 54 to eachother within the piezoelectric element 2, and the ground through-holeconductor 7 for electrically connecting the ground outer electrode 13and ground electrode 43 to each other within the piezoelectric element 2are placed at the longitudinal center position of the piezoelectricelement 2 and arranged in a row in the width direction of thepiezoelectric element 2. Electric connections among electrodes areachieved by the through-hole conductors 6, 7, 8 formed within thepiezoelectric element 2. Therefore, the piezoelectric actuator 1 can bedesigned without taking account of contact with other components, andits components can be made smaller than in the case where sideelectrodes are provided as in conventional multilayer piezoelectricactuators.

The through-hole conductors 6, 7, 8 for connecting the electrodes toeach other are disposed within the piezoelectric element 2 and thus canbe prevented from being affected by their surrounding environments. Thiscan improve the reliability.

When there is a side electrode, a process of forming the side electrodefor each piezoelectric element is necessary after forming the multilayerbody 80 and cutting it into piezoelectric elements during themanufacture. Providing the through-hole conductors 6, 7, 8 within thepiezoelectric element 2 can eliminate the side electrode formationprocess after cutting the multilayer body 80.

Through the ground electrode 43, the first and fourth electrodes 32, 52diagonally positioned with respect to the longitudinal center positionof the piezoelectric element 2 are electrically connected to each other,while the second and third electrodes 34, 54 positioned similarly areelectrically connected to each other. Therefore, the multilayerpiezoelectric actuator 1 causes a longitudinal vibration in thelongitudinal direction and a bending vibration in the thicknessdirection with their node located at the center position of thepiezoelectric element 2 (where the center line CL is located). Since thethrough-hole conductors 6, 7, 8 are formed at the center positionserving as a node, the multilayer piezoelectric actuator 1 can alleviatestresses acting on the through-hole conductors 6, 7, 8. This can improvethe reliability.

Surroundings of a piezoelectric element portion where a through-holeconductor is disposed become softer in structure. Therefore, providing aplurality of through-hole conductors 6, 7, 8 at the longitudinal centerposition of the piezoelectric element 2 makes the structure of thepiezoelectric element 2 softer in the vicinity of its center position,whereby the piezoelectric element 2 becomes easier to deform.Specifically, as shown in FIG. 9, the displacement (indicated bydisplacement DP1 in FIG. 9) of the multilayer piezoelectric actuator 1in accordance with this embodiment is greater than the displacement(indicated by displacement DP2 in FIG. 9) of a conventional multilayerpiezoelectric actuator P in which no through-hole conductors are formedat the longitudinal center position. This can increase the displacementat the time of driving without enhancing internal stresses acting on thepiezoelectric element 2, thereby improving performances.

The foregoing can improve the reliability and performances of themultilayer piezoelectric actuator 1 while making it smaller.

In the multilayer piezoelectric actuator 1, the glass-containingportions 4A₁, 4B₁ (frictional portions 4A, 4B) are formed on the outersurface of the piezoelectric element 2 such as to project from the outersurface. Therefore, the forming of the glass-containing portions 4A₁,4B₁ (frictional portions 4A, 4B) will not affect the designed size ofthe thickness adjustment layers LY1, LY2 that are inactive portions. Thepiezoelectric element 2 can be prevented from being restricted in sizeby the glass-containing portions 4A₁, 4B₁ (frictional portions 4A, 4B).

It is not necessary for the piezoelectric element to be formed with aconcave portion in the multilayer piezoelectric actuator I as in theconventional multilayer piezoelectric actuator. This can eliminate thevariation in the position of the frictional portion caused by theforming of the concave portion, thereby inhibiting the resonancefrequency from fluctuating in resonance modes (the longitudinalvibration mode and bending vibration mode). As a result, the drivingstate of the multilayer piezoelectric actuator 1 can be kept fromvarying.

The frictional portions 4A, 4B have the protective film 5, made of amaterial harder than the glass-containing portions 4A₁, 4B₁, coveringthe glass-containing portions 4A₁, 4B₁. The protective film 5 containsDLC, TiN, SIC, or BP. This can protect the glass-containing portions4A₁, 4B₁ against damages and the like and prevent the glass-containingportions 4A₁, 4B₁ from peeling, thereby appropriately transmitting thedisplacement of the piezoelectric element 2 to the body to be driven 3.

In the multilayer piezoelectric actuator 1, the glass material containedin the glass-containing portions 4A₁, 4B₁ is dispersed in thepiezoelectric element 2. This can improve the connection strengthbetween the piezoelectric element 2 and the glass-containing portions4A₁, 4B₁, thereby preventing the glass-containing portions 4A₁, 4B₁(frictional portions 4A, 4B) from peeling and so forth.

Though a preferred embodiment of the present invention is explained inthe foregoing, the present invention is not necessarily restricted tothe above-mentioned embodiment and can be modified in various mannerswithin a scope not deviating from the gist thereof.

For example, the form and size of the frictional portions 4A, 4B(glass-containing portions 4A₁, 4B₁) are not limited to thoseillustrated in the above-mentioned embodiment. For example, each of theglass-containing portions 4A₁, 4B₁ may have a substantially trapezoidalcross section when cut along a plane orthogonal to the longitudinaldirection of the piezoelectric element 2, i.e., a flat top, as shown inFIG. 10.

When the glass-containing portion formation process S8 provides theglass paste, the top height of thus provided glass paste fluctuates,which is reflected in variations in the height of the glass-containingportions 4A₁, 4B₁. When the height of the glass-containing portions 4A₁,4B₁ varies, the portion of the protective film 5 corresponding to thehighest position of the glass-containing portions 4A₁, 4B₁ comes intopoint contact with the body to be driven 3. Therefore, frictional forcesmay concentrate on the above-mentioned portion of the protective film 5,thereby drastically advancing abrasion.

After forming the glass-containing portions 4A₁, 4B₁, the top portionsof the glass-containing portions 4A₁, 4B₁ are polished flat. Since thetop portions of the glass-containing portions 4A₁, 4B₁ are flat and thushave an even height, the protective film 5 and the body to be driven 3come into surface contact with each other. This can disperse regionswhere frictional forces occur in the protective film 5, therebyinhibiting abrasion from advancing.

When polished flat, the top portions of the glass-containing portions4A₁, 4B₁ have angular corners. When the top portions of theglass-containing portions 4A₁, 4B₁ have angular corners, chipping mayoccur in the glass-containing portions 4A₁, 4B₁ upon a slight shock.Therefore, it will be preferred if the corners of the top portions ofthe glass-containing portions 4A₁, 4B₁ are rounded by polishing (e.g.,mechanical polishing or buffing).

Though the electrode layer (outer electrode layer 11) located on theouter surface of the piezoelectric element 2 and the electrode layers(relay electrode layer 21, first inner electrode layer 31, first groundelectrode layer 41, and second inner electrode layer 51) located withinthe piezoelectric element 2 are electrically connected to each other bythe through-hole conductors 6, 7, 8 positioned within the piezoelectricelement 2, this is not restrictive. For example, an electrode includedin an electrode layer located within the piezoelectric element 2 may bedrawn so as to be exposed at the outer surface of the piezoelectricelement 2, and the electrode portion thus drawn to be exposed at theouter surface of the piezoelectric element 2 and the electrode layerlocated on the outer surface of the piezoelectric element 2 may beelectrically connected to each other through a conductor disposed on theouter surface of the piezoelectric element 2. Meanwhile, the protectivefilm 5 is formed on the outer surface of the piezoelectric element 2 inthis embodiment. When the protective film 5 contains an electricallyconductive material, it will be preferred if a structure achievingelectric connections by the through-hole conductors 6, 7, 8 is employedin order to prevent short circuits from occurring as well.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims.

1. A piezoelectric actuator comprising: a piezoelectric element adaptedto simultaneously generate first and second vibration modes in responseto a voltage applied thereto; and a frictional portion arranged on anouter surface of the piezoelectric element and adapted to come intocontact with a body to be driven and cause a frictional force therewith;wherein the frictional portion has a glass-containing portion containinga glass material and being formed such as to project from the outersurface of the piezoelectric element.
 2. A piezoelectric actuatoraccording to claim 1, wherein the frictional portion further has aprotective film made of a material harder than the glass-containingportion, the protective film covering an outer surface of theglass-containing portion.
 3. A piezoelectric actuator according to claim2, wherein the protective film contains DLC, TiN, SiC, or BP.
 4. Apiezoelectric actuator according to claim 1, wherein the glass materialcontained in the glass-containing portion is dispersed in thepiezoelectric element.